The present invention relates generally to field assemblies for permanent magnet direct current dynamoelectric machines.
Generally speaking, d-c dynamoelectric machines (e.g., motors, and generators or alternators) may be classified as a wound field type or a permanent magnet field type. The permanent magnet type in turn may be further categorized as being of a type that utilizes magnets formed of alloy metals (such as an ALNICO alloy of the General Electric Company); or of a type that utilizes ceramic magnets, these also being called ferro-magnetic ceramic or ferrite magnets.
At one time, ALNICO magnets were used almost universally in d-c rotating machinery; but ceramic materials are now being applied in more and more cases. In the case of permanent magnet direct current motors, one of the reasons for using ceramic rather than ALNICO magnet materials is that ceramic magnets generally exhibit greater resistance to demagnetization and cost less.
In some prior permanent magnet motor designs, the pole faces of ceramic magnets have established one side of the working air gap. With such an approach, different techniques may be utilized to hold the magnets in place relative to the air gap. For example, the magnets may be trapped within performed slots established by a number of aluminum lamina that together establish a cage-like structure (for example, see Herron U.S. Pat. No. 3,671,787).
In other applications, permanent magnets may be adhesively secured to other parts of the field structure (for example, see Susdorf et al U.S. Pat. No. 3,562,568 and Means U.S. Pat. No. 3,772,546); or molten materials such as aluminum may be poured around portions of such magnets so as to partially entrap them (for example, see Eberline et al U.S. Pat. No. 3,368,275).
Unfortunately, motors made pursuant to the above mentioned approaches often are not sufficiently rugged to withstand shock loads or other mechanical and thermal stresses without sustaining damage to one or more magnetics which in turn may cause motor failure.
To be more specific, many ceramic permanent magnet motors use arch or arc-like magnet segments as shown for example in the above mentioned Means patent. As pointed out by Means, the ceramic magnet materials are relatively fragile and brittle and may crack or break due to thermally or mechanically induced stresses. In addition to failure in this mode, ceramic magnets may "delaminate"; i.e., tend to peel apart in layers. It will be understood that breaking, chipping, or peeling of parts of the magnets results in loss of the very material that is relied upon for the magnetic field, and thus motor performance will deteriorate as portions of the magnets break or peel away. Of course, catastrophic failure can also result if magnetic material of sufficient quantity should become lodged in the working air gap.